Here are some academic alternatives to Wikipedia. They are not a substitute for journal articles, but they can be used as an extension of your textbook.
Books on leadership are listed in the Creighton Libraries Information Catalog (CLIC):
Annual Review of Psychology
Avolio, B. J., Walumbwa, F. O., & Weber, T. J. (2009).
Leadership: Current theories, research, and future directions.
Annual Review of Psychology, 60, 421-449.